Coverings used over vehicle airbags often include a tear seam to pre-determine the location of an airbag deployment opening formed through the covering during airbag deployment. Such a tear seam usually includes a weakened portion in the form of a notch, groove, cut, or score formed in a surface of the covering. This weakened portion typically forms an outline or perimeter of the pre-determined deployment opening location. For instance, a score line may be formed in a surface of the covering so that the path formed by the score line forms a shape that generally corresponds in shape and location with an underlying airbag door. When the airbag deploys, the covering tears along the score line to form the deployment opening with the perimeter of the opening at the location of the score line.
It may be more difficult to form tear seams in certain coverings than in others, depending on the type of covering materials, the dimensions of the covering, or the process used to form it. U.S. Pat. No. 7,128,334 to Leland et al. recognizes certain problems with forming tear seams in soft or elastomeric materials, including a tendency for such materials to self-heal. Leland identifies the angle of the sidewalls of the tear seam groove as a critical factor and purports to address that problem by cutting a tear seam with an ultrasonic knife. The knife is shaped to form a groove having a particular V-shaped cross-section to prevent the self-healing phenomenon. Leland does not address difficulties posed by relatively thin covering materials or alignment of the tear seam with other components.